Once an oil or gas well has been formed it is common to install completion infrastructure in the well to control production of hydrocarbons fluids from a hydrocarbon-bearing formation surrounding the well to surface. The completion infrastructure may comprise a string of downhole tools joined by a string of production tubing to surface. The downhole tools are generally flow control or circulation devices such as packers, injection sleeves, production sleeves and the like. Such downhole tools are generally activated mechanically using a shifting tool attached to a work string to open, close or otherwise shift the position of sliding sleeves.
Typically a shifting tool is run on a work string through the completion infrastructure to mechanically actuate the various downhole tools in a desired sequence. In order to mechanically actuate a downhole tool, the shifting tool is manipulated (via the work string) from surface. Typically, actuation is achieved by locking the shifting tool onto profiles provided on the downhole tools and performing a combination of the following operations: pulling (work string in tension) pushing (work string in compression), jarring or rotating to deliver the necessary force or impact to the tool with which it is engaged.
As will be appreciated, it can be difficult to accurately control the operations of the shifting tool especially when it is situated at the end of several kilometers of work string and/or the shifting tool is located in a horizontal or highly deviated wellbore. In these situations it is usually not possible to accurately predict at surface whether the intended actuation has been successful. An additional disadvantage of these conventional shifting tools is the difficultly of use. For example, jarring down or slacking off to cause compression of the work string risks that the work string will ‘catch’ on other downhole tools or land on an unintended component with some force thereby causing damage.
In view of the aforementioned problems with the use of conventional shifting tools, the Applicant developed a method for operating a downhole tool described in co-pending UK patent application no. 1205985.3, in which a work string is first run into a well without actuating any downhole tools. The work string is then used to operate a plurality of downhole tools in a desired sequence as it is pulled out of the well whilst being maintained in tension. This ensures that an operator at surface always has a positive indication of the location of the shifting tool and an accurate log of the operations performed using the shifting tool since every action requires a positive step in order to perform a subsequent operation. Such a system does not, however, provide positive confirmation that the shifting tool has actually actuated or shifted a sliding sleeve of a particular downhole tool. Successful operation of such a system requires an accurate log to be kept of the actuation of all sliding sleeves as the work string is pulled out of the well.